'Sickening' atrocities in Bucha, nearly 70% of Russian troops near Kyiv have withdrawn: Pentagon update Day 40

The Pentagon has been providing daily updates on the Russian invasion of Ukraine and Ukraine's efforts to resist.

Here are highlights of what a senior U.S. defense official told reporters Monday on Day 40:

Nearly 70% of Russian troops near Kyiv have withdrawn

About two-thirds of the Russian forces that were arrayed against Kyiv have withdrawn toward Belarus, according to the official. This is up from an estimated 20% late last week.

Before repositioning, there were close to 20 Russian battalion tactical groups (BTGs) bearing down on Kyiv from the north and northwest, with each group comprised of 700-900 troops. Roughly 13 of those BTGs are now either in Belarus or on their way there, the official said.

The Pentagon believes these forces will be resupplied and possibly reinforced in the north before heading back into Ukraine to fight elsewhere.

"Our best assessment -- and it is only an assessment -- is that they will be applied in the eastern part of the country in the Donbas region," the official said.

During a briefing Monday afternoon, Pentagon press secretary John Kirby said Ukrainian troops are pursuing the Russians as they retreat.

"As they're moving out, Ukrainians are attacking them," Kirby said. "And I'm not a military strategist or a historian, but my understanding of a retreat is that you're moving back as you're getting pushed back, and that's what's happening. So I think that's a very accurate description of what's going on."

The U.S. has also seen some Russian troops leave the Ukrainian city of Sumy to head north to the Russian border, according to the official.

Despite these movements, the official said the "vast majority" of the more than 125 BTGs that Russia committed to the invasion are still inside Ukraine.

'Sickening' Russian atrocities in Bucha

The official said that while the U.S. cannot independently verify Ukrainian claims of Russian atrocities committed against civilians in Bucha, a suburb northwest of Kyiv recently retaken by Ukrainian troops, there is no reason to doubt them.

"It's sickening, it's disgusting," the official said, adding that the claims should be included in the growing list of alleged Russian war crimes to be investigated.

On Monday, Ukrainian President Volodymyr Zelenskyy visited the town, where he accused Russia of genocide. Ukrainian officials have said more than 400 civilians were found dead there, many with hands tied behind their backs, shot at close range.

"We have long said that the Russians would be brutal in their execution of this war. They have been," the senior U.S. defense official said. "We said more than a week ago that we clearly believe the evidence pointed to war crimes by Russian forces. And what we're seeing out of Bucha certainly reinforces those concerns."

Kirby went a step further his briefing.

"It's fairly obvious not just to us but to the world that Russian forces are responsible for the atrocities and Bucha," he said. "Now exactly who, what units, whether they're contractors or Chechens, I don't think we're able to say right now."

Defense Secretary Lloyd Austin "expressed outrage at the apparent atrocities that were committed by Russian forces in Bucha" on a call with his Ukrainian counterpart Oleksii Reznikov, according to Kirby. Austin also committed to using "every tool available" to help document and share information to help hold responsible parties to account.

President Joe Biden put blame on directly on Russian President Vladimir Putin while speaking to reporters Monday.

"You may remember I got criticized for calling Putin a war criminal," Biden said. "Well, the truth of the matter, you saw what happened in Bucha ... he is a war criminal."

Russia shifting airstrikes

In addition to moving some of its troops away from the capital, Russia has also refocused its long-range strikes elsewhere, largely in the eastern and southern parts of the country, the official said.

"Clearly they're still hitting Mariupol, but we have not seen the same level of airstrike activity on Kyiv," the official said. "So there's been a declination there over the last few days."

Russia has fired more than 1,400 missiles against Ukraine since the beginning of the invasion.

Military aid being rushed into Ukraine

The U.S. has continued to rush small arms, anti-tank and aircraft missiles and medical supplies into Ukraine, and has coordinated shipments from a half dozen other donor nations in the last 24 hours, according to the senior defense official.

"Everything we're doing with respect to Ukraine is being expedited -- everything," the official said.